A subscriber line interface circuit typically requires different power supply levels depending upon operational state. One supply level is required when the subscriber equipment is “on-hook” and another supply level is required when the subscriber equipment is “off-hook”. Yet another supply level is required for “ringing”.
In order to ensure sufficient supply levels, a power supply providing a constant or fixed supply level sufficient to meet or exceed the requirements of all of these states may be provided. Such a solution permits one or more SLICs to use a common power supply for at least two operational states.
One disadvantage of a shared fixed power supply architecture is that excess power is generated and must be dissipated as heat or otherwise wasted when a SLIC is not using a power supply level optimized for its particular operational state or for the particular line conditions. For example, the power supply must be capable of supporting the worst-case scenario such as a maximum subscriber line length provided for by specification. In the event the subscriber line is considerably shorter than the maximum expected length, the SLIC will be required to absorb the excess power. The resulting additional thermal load can be problematic for integrated circuits of the SLIC.
One alternative to sharing fixed power supplies is to provide a tracking power supply for each device. Each tracking power supply varies its supply level in accordance with the requirements of its associated device. This tracking power supply architecture is more power efficient than the shared fixed power supply architecture. Given that every device needs its own tracking power supply, however, the tracking power supply per device architecture may not be economical for a large number of SLICs.